VARIATIONS IN BACTERIAL METABOLISM AND DOC USE ALONG A NUTRIENT GRADIENT IN TEMPERATE LAKES

The input of dissolved organic carbon (DOC) may represent a substantial subsidy to the heterotrophic metabolism of lake plankton. A comparative study was conducted to assess how bacterial metabolism and DOC use vary along carbon and nutrient gradients in temperate lakes. Bacterial growth and respiration rates were quantified in re-growth experiments performed in each of 20 lakes (located in Quebec, Canada). Among these lakes, intrinsic growth rates ranged from 0.1 to 1.4 /d, cell specific respiration rates ranged from 1.1 to 19.1 fgO2/cell/h, and growth efficiencies ranged from 5.7 to 58.3 %. These variations were unrelated to DOC input, however. Growth rate and efficiency were instead positively related to phosphorus availability, the least available macronutrient in all lakes. Specific respiration, in contrast, decreased with increasing phosphorus loads, indicating high maintenance energy requirements in the more oligotrophic lakes, regardless of organic carbon availability. Results from lake comparisons were corroborated by a series of nutrient enrichment experiments, and both approaches suggest that the fate of DOC input to lake microbial communities is strongly dependent on nutrient availability.